The present invention relates to inkjet printing method and apparatus of the type described in our prior U.S. Pat. Nos. 5,969,733, 6,003,980 and 6,106,107, and also in our prior International Patent Applications PCT/IL02/00346 and PCT/IL02/01064, published as International Publications. WO 02/090119 A2 and WO03/059626 A2, respectively, the contents of which patents and application are incorporated herein by reference.
Inkjet printers are based on forming drops of liquid ink and selectively depositing the ink drops on a substrate. The known inkjet printers generally fall into two categories: droplet-on-demand printers, and continuous-jet printers. Droplet-on-demand printers selectively form and deposit the inkjet drops on the substrate as and when demanded by a control signal from an external data source; whereas continuous-jet printers are stimulated by a perturbation device, such as a piezoelectric transducer, to emit a continuous stream of ink drops at a rate determined by the perturbation device.
In continuous-jet printers, the drops are selectively charged and deflected to direct them onto the substrate according to the desired pattern to be printed. In binary-type printer systems, the drops are either charged or uncharged and, accordingly, either reach or do not reach the substrate at a single predetermined position. In a multi-level system, the drops can receive a large number of charge levels and, accordingly, can generate a large number of print positions. Both types of systems generally include a gutter for receiving the ink drops not to be printed on the substrate.
The present invention is particularly applicable to continuous-jet printers and is therefore described below with respect to this application. It will be appreciated, however, that aspects of the invention could also be used in droplet-on-demand printers or in other applications.
Continuous, multi-level deflection inkjet technology exists for about 30 years. It is mainly used for low quality, high speed marking. The basic technology is described in U.S. Pat. No. 4,551,731 for example. The system includes of a row of nozzles each of which emits a continuous stream of separate ink drops. Downstream of each nozzle are multi-level charging and deflecting plates for charging each drop and for deflecting the charged drops to selected locations on the substrate according to the pattern to be printed. The system is controlled by a controller that activates and synchronizes the emission, charging and deflection of the drops, and various motions in the system, in order to print a desired pattern, e.g., graphic information, alphanumerical characters, or a combination of both. Each nozzle covers a given line section on the substrate or printing plane.
There are several mechanical configurations for continuous inkjet (CIJ) printing heads. U.S. Pat. No. 4,551,731 describes a configuration wherein the printing drops are deflected to one side (mono-polarity), and non-printing drops fall without deflection (free fall) to the gutters which are located immediately under the nozzles (FIG. 1). Another configuration appears in U.S. Pat. No. 4,395,716 wherein the printing drops are deflected to both sides of the nozzles axis (bi-polarity) to define a line section located under the nozzle, while the non-printing drops are deflected to a gutter located far to the side. In a special implementation (FIG. 2), the gutters are placed on the grounded deflection plates, enabling very large deflections for the printing drops.
The above two multi-level deflection (MLD) configurations are generally characterized by several drawbacks, particularly the following:
1. The mono-polar configuration requires large deflections to one side only. In order to cover large line sections, it is necessary to apply very large charges to the drops. This causes problems of electrostatic interactions between drops in the air. Additionally, it is impossible to use the free falling drops for calibrating the system as they end up in the gutter and not on the printing plane.
2. The bi-polar configuration overcomes some of the problems mentioned above. However, since the gutter drops are heavily charged, they may have interactions with the writing drops. Moreover, because of the extreme position of the gutters, in case of even a small system malfunction the gutter drops may miss the gutters and either hit the deflection plates causing electrical shorts, and/or hit the printed substrate causing a major printing failure.
3. In both configurations, any printing defects in a nozzle will appear in the same relative location on the substrate, and will therefore affect the printing quality.
As there is a relative motion between the print head and the substrate, each nozzle repeatedly prints short line sections of data. For each graphic combination of such a line section, there is a corresponding combination of charging voltages, designed to bring each droplet to its required position on the substrate. The object of many patents is to improve the design of these voltage combinations in order to improve the printing accuracy. Because of electrostatic and aerodynamic interactions between the drops, this task is very complicated. U.S. Pat. Nos. 4,054,882, 4,395,716, 4,525,721, 4,472,722 all deal with methods for the separation and staggering of drops in the air, in order to minimize the interactions between them. However, because of these interactions and other factors in the system, it is very difficult to avoid errors in droplet placement, resulting in printing errors on the substrate.